1. Technical Field
The present disclosure relates to a display device which causes an optically modulated light flux, and a light flux diameter expanding element which expands a light flux diameter.
2. Related Art
A retina scanning display device which causes an optically modulated light flux to enter the eye of the user is provided with a light source unit, a scanning optical system, and a light guide system. The light source unit emits a light flux for displaying an image, the scanning optical system scans the light flux which is emitted from the light source unit, and the light guide system reflects the light flux which is scanned by the scanning optical system to cause the light flux to enter the eye of the user. In the retina scanning display device, when the light flux is small, since the light flux does not enter the pupil when the position of the pupil changes, cases occur in which the image is missing or the like. Therefore, the retina scanning display device is provided with a light flux diameter expanding element (a pupil expanding element).
Meanwhile, a light flux diameter expanding element (a pupil expanding element) is proposed in which two diffraction gratings (a first diffraction grating and a second diffraction grating) face each other, and the diffraction angles thereof are matched by rendering the grating periods of both of the diffraction gratings the same (JP-A-7-72422).
However, since the polarization of the light flux is not considered in the light flux diameter expanding element described in JP-A-7-72422, there is a problem in that when the grating period is small, the light flux diameter may not be suitably expanded. In other words, in a diffraction grating in which concave portions or convex portions which extend linearly are arranged periodically, although incident polarization dependence of the diffraction efficiency increases and the diffraction efficiency increases in relation to polarized light (a Transverse Electric Wave (a TE wave)) in which the electric field vector oscillates in the direction in which the concave portions or the convex portions extend, the diffraction efficiency decreases in relation to polarized light (a Transverse Magnetic Wave (a TM wave)) in which the electric field vector oscillates in a direction perpendicular to the direction in which the concave portions or the convex portions extend. Therefore, when the light flux which is incident on the diffraction grating is a TM wave, the effect of expanding the light flux diameter is reduced, and when the light flux which is incident on the diffraction grating has random polarization, it becomes difficult to obtain a sufficient effect of expanding the light flux diameter.